Quantifying the contribution of audibility to recognition of compression-amplified speech

The speech-recognition advantages of wide-dynamic range compression (WDRC) hearing aids are believed to be a direct result of improved audibility. The main objective of this study was to compare the relationship between increasing audibility and recognition for compression-amplified versus linearly amplified speech, using a quantitative measure of audibility. A second objective was to explore the adequacy of the Aided Articulation Index (Stelmachowicz, Lewis, Kalberer, & Creutz, 1994) in describing performance with WDRC-amplified speech.

Sixteen listeners with mild to severe sensorineural hearing loss were tested on recognition of nonsense syllables that had been digitally processed with linear or WDRC amplification. Speech recognition scores were obtained for three input levels that differed in audibility relative to the listeners' hearing thresholds. An Aided Audibility Index (AAI), ranging from 0.0 (inaudible) to 1.0 (fully audible), was calculated for each listener and condition. This importance-weighted index represents the portion of the speech signal that is available to the listener. The relationship between audibility and recognition was then examined for each type of amplification.

At low and moderate input levels, AAI values and corresponding recognition scores were higher for the compression-amplified than for the linearly amplified speech. At high input levels, AAIs and recognition scores were essentially the same for both types of amplification. Recognition scores increased monotonically as a function of the AAI for both linear and WDRC amplifiers. There was no significant difference between the functions for linearly amplified versus compression-amplified speech. In other words, a given increase in audibility resulted in the same increase in recognition for both types of amplification.

Increasing the amount of audible speech information plays the same role in recognition for compression-amplified as for linearly amplified speech. This suggests that, at least for the conditions tested here, compression does not introduce detrimental changes to the speech signal that offset the benefits of improved audibility. Accuracy of the compression-amplified AAI can be improved by substituting measured speech ranges for the estimated speech ranges used in the standard AAI calculation. In the absence of measured speech ranges, the standard AAI provides a close approximation.